Symmetric LED array for pulse oximetry

ABSTRACT

A sensor for pulse oximeter systems is provided which comprises a first source of electromagnetic radiation configured to operate at a first wavelength, a second source of electromagnetic radiation configured to operate at a second wavelength and a third source of electromagnetic radiation configured to operate at a third wavelength. The first and third sources of electromagnetic radiation are symmetrically oriented about an axis.

BACKGROUND

1. Field of Invention

The present invention relates generally to medical devices and, moreparticularly, to sensors used for sensing physiological parameters of apatient.

2. Description of the Related Art

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present invention,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

In the field of medicine, doctors often desire to monitor certainphysiological characteristics of their patients. Accordingly, a widevariety of devices have been developed for monitoring many suchcharacteristics of a patient. Such devices provide doctors and otherhealthcare personnel with the information they need to provide the bestpossible healthcare for their patients. As a result, such monitoringdevices have become an indispensable part of modern medicine.

One technique for monitoring certain physiological characteristics of apatient is commonly referred to as pulse oximetry, and the devices builtbased upon pulse oximetry techniques are commonly referred to as pulseoximeters. Pulse oximetry may be used to measure various blood flowcharacteristics, such as the blood oxygen saturation of hemoglobin inarterial blood, the volume of individual blood pulsations supplying thetissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to thetime varying amount of arterial blood in the tissue during each cardiaccycle.

Pulse oximeters typically utilize a non-invasive sensor that transmitsor reflects electromagnetic radiation, such as light, through apatient's tissue and that photoelectrically detects the absorption andscattering of the transmitted or reflected light in such tissue. One ormore of the above physiological characteristics may then be calculatedbased upon the amount of light absorbed and scattered. Morespecifically, the light passed through or reflected from the tissue istypically selected to be of one or more wavelengths that may be absorbedand scattered by the blood in an amount correlative to the amount ofblood constituent present in the tissue. The measured amount of lightabsorbed and scattered may then be used to estimate the amount of bloodconstituent in the tissue using various algorithms.

Certain events can create error in these measurements. For example,pulse oximetry measurements may be sensitive to movement of the sensorrelative to the patient's tissue, and various types of motion may causeartifacts that may obscure the blood constituent signal. Specifically,motion artifacts may be caused by moving a sensor in relation to thetissue, increasing or decreasing the physical distance between emittersand detectors in a sensor, changing the angles of incidents andinterfaces probed by the light, directing the optical path throughdifferent amounts or types of tissue, and by expanding, compressing, orotherwise altering tissue near a sensor.

Pulse oximetry may utilize light sources that emit in at least twodifferent or spectral regions, one that emits in the red region(typically about 660 nm) and one in the near infrared region (typicallyabout 890-940 nm). Typically, LEDs are used as light sources and areheld in close proximity, i.e., optically coupled, to a tissue locationbeing probed. In the context of pulse oximetry, optical coupling refersto a relationship between the sensor and the patient, permitting thesensor to transmit light into the patient's blood profused tissue andpermitting a portion of the light to return to the sensor after passingthrough or reflecting from within the tissue. The quality of the opticalcoupling of the emitters and detectors is related to the amount of lightthat actually enters the patient's tissue and the portion of the lightreceived by the sensor that passes through the patient's blood profusedtissue. As described earlier, motion and/or the application of excessivepressure can have the effect of changing the relative optical couplingefficiency of the light sources and the detector. Even when two LEDs aremounted side by side, motion induced changes in optical efficiency haveresulted in distortions of the photoplethysmographs produced by the twoLEDs. The result of poor coupling, therefore, is a decrease in theaccuracy of the sensor.

Homogenizing the light sources using optical coupling devices is one wayof mitigating the effect of motion-induced changes in optical efficiencyon the accuracy of a pulse oximeter. Such techniques, however, generallyrequire careful optical alignment, tend to be expensive, or reduce theoptical coupling efficiency into the tissue.

Sensor-to-sensor spectral variation of light sources used for oximetersensors may also affect a pulse oximeter's accuracy. Because hemoglobin(HbO₂ and HHb) spectra vary more rapidly as a function of wavelength atapproximately 660 nm than at approximately 940 nm, the precise spectralcontent of the 660 nm light source is more critical. Currentmanufacturing processes used to produce 660 nm LEDs result in a widedistribution of spectral content, potentially necessitating modificationof the calibration model according to actual spectral content of the 660nm source, thus adding cost to the system. Alternatively, choosing onlyLEDs that emit in a narrow wavelength range would result in lowproduction yields and higher sensor cost. Thus, costs are incurredeither by limiting the range of wavelengths to reduce the need forcalibration, or by allowing for a wider spectral content and insertingcalibration models.

SUMMARY

Certain aspects commensurate in scope with the originally claimedinvention are set forth below. It should be understood that theseaspects are presented merely to provide the reader with a brief summaryof certain forms the invention might take and, these aspects are notintended to limit the scope of the invention. Indeed, the invention mayencompass a variety of aspects that may not be set forth below.

In accordance with one aspect of the present invention a sensor forpulse oximeter systems is provided. The sensor comprises a first sourceof electromagnetic radiation configured to operate at a firstwavelength, a second source of electromagnetic radiation configured tooperate at a second wavelength and a third source of electromagneticradiation configured to operate at a third wavelength. The first andthird sources of electromagnetic radiation overlap at their half powerlevel or greater and correspond to a center wavelength in the range of650 to 670 nm. A photodetector is configured to receive electromagneticradiation from blood-perfused tissue irradiated by the first, second andthird sources of electromagnetic radiation.

In accordance with another aspect of the present invention there isprovided a sensor comprising a first source of electromagnetic radiationconfigured to operate at a first wavelength, a second source ofelectromagnetic radiation configured to operate at a second wavelengthand a third source of electromagnetic radiation configured to operate ata third wavelength. A photodetector is configured to receiveelectromagnetic radiation from the blood-perfused tissue, and the firstand third sources of electromagnetic radiation are symmetricallydisposed spatially relative to the photodetector.

In accordance with yet another aspect of the present invention a sensorcomprising a first light emitting diode configured to emit radiationhaving a maximum intensity corresponding to wavelengths in a red regionof the electromagnetic spectrum. The sensor also comprises a second LEDconfigured to operate in the near-infrared region of the electromagneticspectrum and a third LED configured to operate in the red region of theelectromagnetic spectrum. The third LED has a maximum intensity at awavelength greater than 650 nm and greater than the wavelength at whichthe first LED has a maximum. The first LED and third LED are spectrallysymmetrical with respect to a center wavelength in the range 650 to 670nm.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent upon reading thefollowing detailed description and upon reference to the drawings, inwhich:

FIG. 1 illustrates a block diagram of a pulse oximeter system inaccordance with an exemplary embodiment of the present invention;

FIG. 2 illustrates spatial symmetry of the light sources in accordancewith an exemplary embodiment of the present invention:

FIG. 3 illustrates an emission intensity plot of an emitter inaccordance with an embodiment of the present invention;

FIG. 4 illustrates the emission intensity plots of two emittersspectrally symmetrical relative to a central wavelength in accordancewith embodiments of the present invention;

FIG. 5 illustrates an electrical configuration for LEDs of a pulseoximeter in accordance with an exemplary embodiment of the presentinvention; and

FIG. 6 illustrates a cross-sectional view of a pulse oximeter sensor inaccordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve developer's specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

In accordance with aspects of the present invention, techniques aredisclosed for reducing the susceptibility of pulse oximeters to errorcaused by motion or spectral variation of light sources. Additionally,techniques are disclosed that allow for the operation of pulse oximetrysystems with a broad spectral content and, potentially, withoutcalibration.

Turning to FIG. 1, a block diagram of a pulse oximeter system inaccordance with an exemplary embodiment of the present invention isillustrated and generally designated by the reference numeral 10. Thepulse oximeter system 10 includes a sensor 11 having a detector 12 whichreceives electromagnetic radiation from the blood perfused tissue of apatient 14. The electromagnetic radiation originates from emitters 16. Aphotoelectric current is generated when the electromagnetic radiationscattered and absorbed by the tissue arrives at the detector 12. Thecurrent signal produced by the detector 12 is amplified by an amplifier18 prior to being sent to the pulse oximeter 20.

The emitters 16 may be one or more LEDs configured to emit in the redand near infrared regions of the electromagnetic spectrum. As will beexplained in greater detail below, the emitters 16 may be oriented toprovide spatial symmetry about an axis. Additionally, the emitters 16may be spectrally symmetrical about a central wavelength to eliminatethe use of a spectrum calibration model.

In addition to providing a signal corresponding to the amount ofelectromagnetic radiation scattered and absorbed by the tissue, thesensor 11 may also be configured to provide calibration data to thepulse oximeter 20 via an encoder 21. Pulse oximetry algorithms typicallyuse coefficients indicative of certain parameters of a particularsystem. The particular set of coefficients chosen for a particular setof wavelength spectra is determined by the value indicated by encoder 21corresponding to a particular light source in a particular sensor. Inone configuration, multiple resistor values may be assigned to selectdifferent sets of coefficients. In this instance, the encoder 21 mayinclude one or a plurality of resistor values and a detector 22 locatedin the pulse oximeter 20 reads the resistor values and selectscoefficients from a stored table. Alternatively, the encoder 21 may be amemory that either stores the wavelength information or thecoefficients. Thus, the encoder 21 and the decoder 22 allow the pulseoximeter 20 to be calibrated according to the particular wavelengths ofthe emitters 16.

In an exemplary embodiment, the pulse oximeter 20 includes amicroprocessor 24 that processes data received from the sensor 11 tocompute various physiological parameters. The pulse oximeter 20 may alsoinclude a random access memory (RAM) 26 for storing data and an outputdisplay 28 for displaying computed parameters. A time processing unit(TPU) 30 may be provided to control the timing of the pulse oximeter 20.The TPU may be coupled to light drive circuitry 32 and a switch 34. Thelight drive circuitry 32 controls light emissions of the emitters 16,and the switch 34 receives and controls the gating-in of the amplifiedsignals from the detector 12. The received signals are passed from theswitch 34 through a second amplifier 36, a filter 38, and ananalog-to-digital converter (A/D) 40, before arriving at themicroprocessor 24.

Upon receiving the signals, the microprocessor 24 calculates the oxygensaturation and other physiological parameters. In calculating thephysiological parameters, the microprocessor 24 uses algorithms storedon a read-only memory (ROM) 44 and data stored in the RAM 26. Asdiscussed above, the algorithms typically use coefficients whichcorrespond to the wavelengths of light used and calibrate the pulseoximeter 20 to the particular wavelengths being used. Implementation ofspectral symmetry techniques may, however, eliminate such calibration.

The display 28 outputs the physiological parameters, such as oxygensaturation, calculated by the pulse oximeter 20. The block diagram ofthe pulse oximeter system 10 is exemplary, and it should be understoodthat various alternative configurations may be possible. For example,there may be multiple parallel paths of separate amplifiers, filters,and A/D converters for multiple light wavelengths or spectra received.Additionally, implementation of spectral symmetry techniques may obviatethe encoder 21 and decoder 22.

Spatial symmetry of the emitters 16 may provide a level of immunityagainst motion induced artifacts. FIG. 2 diagramatically illustrates theemitter portion of a sensor having emitter LEDs 64, 65 and 66symmetrically oriented relative to an axis 62 in accordance with anexemplary embodiment of the present invention. The first LED 64 ispositioned on one side of the center LED 65, while the second LED 66 ispositioned on the other side of the center LED 65. The axis 62 runsthrough the center of the center LED 65 and may represent the long axisof a patient's finger to which the LEDs may couple. The center LED 65typically emits radiation in the infrared (IR) or near infrared (NIR)range, while the LEDs 64 and 66 have similar spectral outputs in the redrange of approximately 600 to 800 nm to help ensure that any couplingissues that may occur due to movement of tissue relative to one LED maybe compensated for by the other LED. For example, if the finger movesaway from the LED 64, resulting in poor coupling with LED 64, thecoupling of the finger with LED 66 may still exhibit good coupling oreven improved coupling due to the movement.

As discussed above, pulse oximeters typically employ light sources thatoperate in the near infrared (NIR)/infrared (IR) and the red range ofthe electromagnetic spectrum. The different wavelengths of lightgenerate different levels of current in the photodetector 12. As the redrange produces a lower photocurrent in the photodetector 12, LEDs thatemit in this range may be selected as the LEDs 64 and 66. Because thesignal from the two LEDs 64 and 66 is additive, the signal-to-noiseratio of the sensor may be increased, thus, providing better readings.

In addition to the LEDs 64 and 66 being physically disposed in asymmetrical relationship, or in the alternative, the wavelengths of theLEDs 64 and 66 may be spectrally symmetrical with respect to oneanother. Spectral symmetry of the LEDs 64 and 66 may be implemented incombination with or independent from the spatial symmetry describedabove. FIG. 3 illustrates the emission intensity plot of an exemplarylight emitter relative to the wavelength. Specifically, FIG. 3illustrates that the emitter exhibits an emission intensity maximum at acenter wavelength λ_(c). The wavelengths λ₆₄ and λ₆₆ of the LEDs 64 and66 are shown symmetrically disposed about the center wavelength λ_(c).To provide spectral symmetry, two emitters are used that have emissionintensity maxima at wavelengths equidistant in nanometers from thecentral wavelength λ_(c) and on opposite spectral sides of the centralwavelength λ_(c), which may be selected to be 660 nm, for example.Specifically, the two wavelengths λ₆₄ and λ₆₆ are selected to havemaxima at wavelengths that overlap at their half power level or greaterat the center wavelength λ_(c) such that when summed together theyachieve a maximum intensity at the center wavelength λ_(c), where themaximum intensity is greater than that of either LED 64 or LED 66 alone.For example, if the spectral bandwidth of the wavelengths λ_(c), λ₆₄ andλ₆₆ are the same, two wavelengths λ₆₄ and λ₆₆ may be selected tocorrespond to the half power level or greater of the center wavelengthλ_(c). In other words, if the LED 64 has a maximum at 650 nm and the LED66 has a maximum at 670 nm and the respective signals have not decreasedbeyond their half power level (−3 dB) at 660 nm, then the additivemaximum of the LED 64 and LED 66 will occur at 660 nm. Thus, a strongersignal at the center wavelength, such as 660 nm may be achieved throughspectral symmetry techniques.

Additionally, the use of spectral symmetry may eliminate the need for acalibration model. The hemoglobin (HbO₂ and HHb) spectra vary morerapidly as a function of wavelength at 660 nm than at 940 nm. Therefore,the precise spectral content of the red light source is more criticalthan that of the NIR/IR light source. Accurate predictions of oxygensaturation may be achieved by modification of the calibration modelaccording to the spectral content of the particular red light sourcesbeing used, as discussed above. Spectral symmetry techniques, however,may be used to obviate calibration.

Referring to FIG. 4, the emission intensity of the two LEDs 64 and 66having maxima at wavelengths λ₆₄ and λ₆₆, which are symmetrical aboutthe center wavelength λ_(c), e.g., approximately 660 nm, areillustrated. A maximum at the center wavelength, indicated by the dashedline 67, occurs due to the additive effects of the LEDs 64 and 66emitting at the spectrally symmetrical wavelengths λ₆₄ and λ₆₆ whichoverlap at their half power level or greater. As discussed above, eventhough the LEDs 64 and 66 are not operating at the center wavelengthλ_(c), they may combine to create a maximum at the center wavelengthλ_(c). Thus, the technique of spectral symmetry may eliminate thewavelength specific calibration because the LEDs 64 and 66 are selectedto be summed to create a maximum at the center wavelength λ_(c) forwhich the pulse oximeter may already be programmed. It is inherent inthis technique that the wavelength maximum, and not the spectral widthof the light source, is used for the calibration of oximeter sensorlight sources.

Furthermore, because the light sources are spectrally symmetrical andtheir intensities may add to have a maximum at the center wavelengthλ_(c), a wider range of light source spectra may be used. For examplethe range of currently allowed wavelengths for the 660 nm LEDs isapproximately 650 nm to 670 nm. According to the techniques presentedherein, however, it may be possible to use LEDs emitting outside therange of wavelengths between 650 and 670 nm. For example, a first LEDcan be selected to have an emission peak at a wavelength less than 670nm, such as 648 nm, and second LED may be selected to have an emissionpeak at a wavelength greater than 650 nm, such as 672 nm. As long assignals from the first and second LEDs overlap at half power (−3 dB) orgreater, a peak will be created by the overlap. Assuming that each LEDhas an equivalent spectral bandwidth, there will be a peak at 660 nm.Alternatively, the first LED can be selected to emit at 640 nm and thesecond LED can be selected to emit at 660 nm, thus providing spectralsymmetry at 650 nm. Again, as long as the signals emitted from the firstand second LEDs overlap at half power or greater at 650 nm, there willbe a peak at 650 nm. The use of LEDs producing maximas at wavelengthother than 660 nm, however, may require a calibration model tocompensate for the lack of absorbance of hemoglobin at that particularwavelength. Additionally, the actual range of wavelengths that may beimplemented may be limited by several factors, including the spectralbandwidth of the particular LEDs, the photosensitivity of the detectorand limits on the spectrophotographic response of hemoglobin atwavelengths other than 660 nm. Specifically, if the LEDs only have aspectral bandwidth of twenty nanometers, the spectrally symmetrical LEDscan only have peaks twenty nanometers or less apart (i.e. ten nanometersfrom a desired center wavelength

In an alternative exemplary embodiment, the implementation of thespectral symmetry techniques may produce a peak having a broaderspectral bandwidth, rather than increasing the magnitude of the signalat the center wavelength. Specifically, the peak generated by summingthe emitted wavelengths may not necessarily be greater than the peakgenerated by the individual LEDs 64 and 66 themselves. For example, thesummed peak may have a magnitude approximately equivalent to themagnitude of peaks generated by the LEDs 64 and 66 alone. Accordingly,the intensity of the emissions across the spectra will be relativelyflat between the wavelengths being used and at the center wavelength.The combined signal would provide a broader spectral bandwidth a thecenter wavelength, as the bandwidth extends from half power level on theblue side of the signal from LED 64 to the half power level on the redside of the LED 66.

An exemplary schematic of the electrical configuration of the multipleLEDs is illustrated in FIG. 5. The configuration of the LEDs may be thesame regardless of whether the emitters provide spectral and/or spatialsymmetry. The two LEDs 64 and 66 are electrically configured to emitlight coincidently, whereas the center LED 65 is configured to emitlight while the LEDs 64 and 66 are off.

Turning to FIG. 6, a cross sectional view of a sensor in accordance withan exemplary embodiment of the present invention is illustrated andgenerally designated by the reference numeral 68. The cross sectionalview of the sensor 68 shows a plurality of LEDs 64, 65 and 66 orientedabout an axis 62. The center LED 65 and the detector 16 are bisected bythe axis 62. The LEDs 64, 65, and 66 transmit electromagnetic radiationthrough a patient's tissue 14 which is detected by the detector 16. Thehousing 70 of the pulse oximeter 68 may be designed to limit movement ofthe patient 14 relative to the LEDs 64, 65, 66 and the detector 16,thus, reducing artifacts due to motion and poor coupling of the LEDs 64,65, 66 and detector 16 to the patient. Specifically, the sensor 68includes a curved shape about the patient's tissue 14 which permitsrocking movement according to the curved shape of the housing 70, butwhich limits other movement. Movement such as rocking along thecurvature of the housing 70 may be anti-correlated by the spatialsymmetry of the sensor 68, thus reducing motion-induced artifacts.

As stated above, spatial symmetry techniques may be used in combinationwith or independent of the spectral symmetry technique. When implementedin a system that does not have spectral symmetry, it may be desirable tocalibrate the pulse oximeter. When spectral symmetry techniques areimplemented, the calibration may be unnecessary, as described above. Inthe event that spectral symmetry is implemented and the λ_(c) is not 660nm, it may still be desirable to calibrate according to the particularλ_(c).

Several advantages are achieved by implementing the techniques describedherein. For example, spatial symmetry may provide anti-correlation ofmotion-induced artifacts and increase the signal-to-noise ratio.Motion-induced artifacts are typically a result of changes in thecoupling of the sensor with the patient's tissue. The spatial symmetryanti-correlates the motion induced artifacts by providing two LEDssymmetrically disposed about an axis of movement such that as thepatient's tissue moves away from one LED the tissue couples with anotherLED operating at the same wavelength. Additionally, the summed signalfrom the symmetrically disposed LEDs may provide a stronger signal thana single LED to improve the signal-to-noise ratio for wavelengths whichhave a weaker photodetection effect.

The implementation of spectral symmetry may also provide a strongersignal at wavelengths which have a weaker photo detection effect. Thecombined emission strength of the two LEDs spectrally oriented about acentral wavelength may provide a stronger signal for detection if eachof LEDs have emission wavelengths which overlap above their half powerlevel, as described above with reference to FIG. 4. Furthermore, thespectral symmetry allows the use of LEDs having a wider range ofspectral content. Selecting light sources having maxima symmetricallydisposed about a center wavelength in the range of 650 to 670 nm allowsfor a summed signal with a maximum within the 650 to 670 nm range. Thus,LEDs emitting outside of the 650 to 670 nm range may be used when pairedwith an LED having a peak emission wavelength symmetrically disposedabout the center wavelength, as long as the spectra of the LEDs overlapat the center wavelength at their respective half power levels orgreater.

Implementation of spectral symmetry may also allow for calibration-freesensors. Assuming that the wavelength maximum and not the spectral widthof the LEDs is the most important aspect of the calibration, a centerwavelength can be selected about which LED pairings are spectrallysymmetrical. The pulse oximeter 20 can be set to operate according to acenter wavelength, i.e. according to coefficients associated with thecenter wavelength, and no calibration is required. If variable spectralwidth due to the use of two LEDs is found to limit the accuracy of themeasurement, an optical coating could be applied either to the lightsource or detector to limit the spectral width. For example, thedetector could be coated with a material that passes light only withbands around 660 and 890 nm, but blocks the detection of light in allother spectral regions. In this way, the detected spectral band widthwould be primarily determined by the spectral width of the opticalbandpass filter. This aspect of the invention would have the additionaladvantage of greatly limiting the influence of ambient light on themeasured signal. Examples of suitable coatings include multilayerdielectric films and light-absorbing dyes.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Indeed, the presenttechniques may not only be applied to measurements of pulse oximetry,but these techniques may also be utilized for the measurement and/oranalysis of other blood or tissue constituents. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims. It will be appreciated by those working in the art thatsensors fabricated using the presently disclosed and claimed techniquesmay be used in a wide variety of contexts.

1. A sensor comprising: a substantially annular housing configured to bedisposed about a patient's finger, the annular housing being bisected byan imaginary line into a first semicircular region and a secondsemicircular region; a first source of electromagnetic radiation coupledto the first semicircular region of the annular housing, and configuredto operate at a first wavelength; a second source of electromagneticradiation coupled to the first semicircular region of the annularhousing, and configured to operate at a second wavelength; a thirdsource of electromagnetic radiation coupled to the first semicircularregion of the annular housing, and configured to operate at a thirdwavelength, wherein the second source is positioned between the firstand third sources along a circumference of the annular housing; and aphotodetector coupled to the second semicircular region of the annularhousing, and configured to receive electromagnetic radiation from thefirst, second and third sources after the electromagnetic radiation haspassed through blood-perfused tissue of the patient's finger; whereinthe photodetector is positioned generally opposite from the secondsource along the circumference of the annular housing, the first andthird sources are generally symmetrically disposed about an axis thatbisects the second source and the photodetector, the first, second andthird sources are the only sources of electromagnetic radiation coupledto the annular housing, and the photodetector is the only detectorcoupled to the annular housing.
 2. The sensor of claim 1, wherein thefirst and third sources of electromagnetic radiation emit spectra thatoverlap at a half power level or greater of their respective emissionmaxima, the addition of the spectra causing a peak corresponding to acenter wavelength.
 3. The sensor of claim 1, wherein the first and thirdsources of electromagnetic radiation are configured to emit at a centerwavelength in the range of 650 to 670 nm.
 4. The sensor of claim 1,wherein the second source of electromagnetic radiation is configured toemit in the near-infrared region of the electromagnetic spectrum.
 5. Thesensor of claim 1, wherein the second source of electromagneticradiation is configured to emit in the infrared region of theelectromagnetic spectrum.
 6. The sensor of claim 1, wherein the firstand third sources of electromagnetic radiation emit in the red region ofthe electromagnetic spectrum.
 7. The sensor of claim 1, wherein thefirst and third sources of electromagnetic radiation have peak emissionspectra symmetrically disposed about a center wavelength, the firstsource having a peak emission at less than the center wavelength and thethird source having a peak emission at greater than the centerwavelength.
 8. The system of claim 1, wherein the annular housing isconfigured to allow optical coupling of the patient's finger with thefirst, second and third sources, and the photodetector.
 9. The system ofclaim 1, wherein the annular housing is configured to inhibit axialmovement of the patient's finger.
 10. The sensor of claim 1, wherein thefirst and third sources are circumferentially spaced from the secondsource such that as the patient's finger rotates within the curvedhousing, reduced optical coupling between the patient's finger and thefirst source increases optical coupling between the patient's finger andthe third source, and reduced optical coupling between the patient'sfinger and the third source increases optical coupling between thepatient's finger and the first source.
 11. A method of operating a pulseoximeter comprising: transmitting electromagnetic radiation from a firstlight emitting diode (LED), a second LED and a third LED, wherein eachLED is directed radially inward toward a patient's finger, the secondLED is configured to emit electromagnetic radiation at wavelengths inthe near-infrared or infrared region of the electromagnetic spectrum,the first and third LEDs have maxima at wavelengths in the 600 to 800 nmrange, the first, second and third LEDs are positioned on a first sideof the patient's finger, the second LED is positioned between the firstand third LEDs, a photodetector is positioned generally opposite thesecond LED on a second side of the patient's finger, wherein the firstLED, the second LED, the third LED and the photodetector are coupled toa curved housing, wherein the first, second and third LEDs arecircumferentially separated from each adjacent LED along the curvedhousing, and the curved housing is configured to be disposed about thepatient's finger, the first and third LEDs are generally symmetricallydisposed about an axis that bisects the second LED and thephotodetector, no additional photodetector is directed toward thepatient's finger, no additional LED is directed toward the patient'sfinger, and the first and third LEDs are configured to operatesimultaneously; generating a photoelectrical signal via thephotodetector corresponding to the electromagnetic radiation from thefirst, second and third LEDs; computing physiological parameters basedon the generated signal.
 12. The method of claim 11, wherein the firstand third LEDs are circumferentially spaced from the second LED suchthat as the patient's finger rotates within the curved housing, reducedoptical coupling between the patient's finger and the first LEDincreases optical coupling between the patient's finger and the thirdLED, and reduced optical coupling between the patient's finger and thethird LED increases optical coupling between the patient's finger andthe first LED.
 13. The method of claim 11, wherein the first, second andthird LEDs are coupled to a first semicircular region of the curvedhousing, the photodetector is coupled to a second semicircular region ofthe curved housing, and the first semicircular region does not overlapthe second semicircular region.
 14. A system for determiningphysiological parameters comprising: a sensor comprising: a curvedhousing configured to be disposed about a patient's finger; a firstlight emitting diode (LED) coupled to a first semicircular region of thecurved housing, wherein the first LED is directed radially inward, andconfigured to emit electromagnetic radiation at wavelengths in the redregion of the electromagnetic spectrum; a second LED coupled to thefirst semicircular region of the curved housing, wherein the second LEDis directed radially inward, and configured to emit electromagneticradiation at wavelengths in the near-infrared or infrared region of theelectromagnetic spectrum; a third LED coupled to the first semicircularregion of the curved housing, wherein the third LED is directed radiallyinward, and configured to emit electromagnetic radiation at wavelengthsin the red region of the electromagnetic spectrum, and wherein thesecond LED is positioned between the first and third LEDs along acircumference of the curved housing; and a photodetector coupled to asecond semicircular region of the curved housing, and configured togenerate a signal corresponding to received electromagnetic radiationfrom blood-perfused tissue irradiated by the first, second and thirdLEDs, wherein the photodetector is directed radially inward, thephotodetector is positioned generally opposite from the second LED alongthe circumference of the curved housing, the first semicircular regiondoes not overlap the second semicircular region, the first and thirdLEDs are generally symmetrically disposed about an axis that bisects thesecond LED and the photodetector, the sensor does not include a fourthLED, and the sensor does not include a second photodetector; and a pulseoximeter configured to compute physiological parameters based on thesignal generated by the photodetector.
 15. The system of claim 14,wherein the first and third LEDs are circumferentially spaced from thesecond LED such that as the patient's finger rotates within the curvedhousing, reduced optical coupling between the patient's finger and thefirst LED increases optical coupling between the patient's finger andthe third LED, and reduced optical coupling between the patient's fingerand the third LED increases optical coupling between the patient'sfinger and the first LED.